1. Field of the Invention
The present invention relates to a positioning system utilizing at least four artificial satellites which are placed in geosynchronous altitude orbits and are always visible from a ground control station (GCS) for calculating a position of an observation point by a positioning algorithm by using positioning signals transmitted from the artificial satellites, a positioning system utilizing the artificial satellites having a communication means, and a positioning method utilizing the artificial satellites.
2. Description of the Related Arts
As a conventional positioning system utilizing artificial satellites, for example, a GPS (Global Positioning System) is known. An outline of the GPS is disclosed in "Principle of Operation of NAVSTAR and System Characteristics", by R. J. Milliken and C. J. Zoller, Navigation, Journal of The Institute of Navigation, Vol. 25, No. 2, pp. 95-106, Summer 1987.
FIG. 1 illustrates an arrangement of artificial satellites used in the GPS. Numerals 1310, 1311, 1312 and 1313 designate the artificial satellites as NAVSTARs (Navigation System with Timing and Ranging), and their positions are indicated as A, B, C and D. Each NAVSTAR has a precise and stable atomic frequency standard (atomic clock) and can maintain a high accuracy of generated clock signal and calibrate the clock at a predetermined period by standard time information received from a GCS (Ground Control Station) to maintain the accurate time.
Also, concerning the positions of the NAVSTARs, the GCS carries out the determination and prediction of their orbits by the tracking data of the NAVSTARs and thus, when a time is selected, the positions of the NAVSTARs can be determined on the basis of the ephemerides. Hence, if an observer stays at the NAVSTAR, the position of the observer can be made known from the present time of the NAVSTAR and the orbital elements of its time. However now, a case where an observer remotely observes the NAVSTARs and carries out a determination of his own position will be considered.
In FIG. 1, a numeral 1314 indicates an observation point P of the observer. Now, it is assumed that there is a certain error At between a time of a timing device the observer possesses and a standard time. It is also assumed that the observer carries out the observation at a time Tno +.DELTA.t, wherein Tno is the standard time when the observer carries out the observation and .DELTA.t is an inherent error of the timing device the observer has. Now, assuming that the four NAVSTARs A, B, C and D are observed by the observer at respective times Tn1, Tn2, Tn3 and Tn4, it is considered that signals representing the above-described times reach from the artificial satellites to the observer with delays corresponding to radio wave propagation times between the observer and the respective artificial satellites, and hence from the relationship between the observed times and the positions of the NAVSTARs, the following equations can be obtained: EQU AP=C(Tno+.DELTA.t-Tn1) EQU BP=C(Tno+.DELTA.t-Tn2) EQU CP=C(Tno+.DELTA.t-Tn3) . . . (i) EQU DP=C(Tno+.DELTA.t-Tn4)
wherein AP, BP, CP and DP represent the range between the observation point P and the respective NAVSTARs and C represents the velocity of light.
In these four simultaneous equations (i), Cartesian coordinates x, y and z for defining the position of the observation point P (positioning point) and the error At of the timing device are included as unknown value. Hence, these equations can have no solutions other than a singular point and the observation point can be decided. Also, the inherent error At of the timing device the observer possesses can be known and thus the timing device can be calibrated.
The artificial satellites, NAVSTARs, for the GPS move in circular orbits at an altitude 20,183 Km. There are six orbital planes, and three artificial satellites are arranged at an equal interval in each orbit. Hence, when at least 18 artificial satellites are placed altogether in the six orbits, the positioning available areas can be formed all over the earth.
The circulation period of the NAVSTARs is 12 hours, and the same type of artificial satellites are present in the visible area seen from the GCS for controlling the artificial satellites for a limited time. However, the artificial satellites successively come by turn into the visible area and at least four artificial satellites can always exist in the visible area seen from the GCS.
Further, another conventional positioning system utilizing artificial satellites placed in quasi-geosynchronous altitude orbits having a large orbital inclination angle and a stationary altitude is disclosed in "Positioning Satellite System Using Intersatellite Communication", by K. Inamiya, Journal of Spacecraft and Rockets, AIAA. Vol. 28, No. 6, pp. 720-727, Dec.-Nov. 1991.
FIG. 2 shows this conventional positioning system utilizing artificial satellites. Numerals 1310, 1311, 1312 and 1313 denote artificial satellites A, B, C and D, respectively. Between a GCS 1320 and the artificial satellite A, a feeder link (FL.sub.AG) 1321 is present, and a telemetry tracking command channel (TC.sub.BG) 1322 is shown as one example between the GCS and the artificial satellite B. Numerals 1324, 1326 and 1328 designate respective communication channels (RT.sub.AB, RT.sub.BC and RT.sub.CD) between the adjacent artificial satellites A, B, C and D. The positioning signals (RT.sub.AP, RT.sub.BP, RT.sub.CP and RT.sub.DP) 1321, 1325, 1327 and 1829 are transmitted From the respective artificial satellites A, B, C and D to a predetermined positioning available area.
When an artificial satellite is placed in a quasigeosynchronous altitude orbit having a large orbital inclination angle and a stationary altitude, a figure-eight characteristic is drawn on a ground surface. In this system, in the case of placing four artificial satellites in the orbits, the artificial satellites are arranged so that points intersecting with an equatorial plane of the figure-eight orbit may spread so as to be the space looked at from the observer. In order to form a positioning available area of this positioning system globally, 12 artificial satellites are placed in the orbits and are arranged at an interval of 30 degrees longitude in the equatorial plane, and the time relationship is determined so that mean anomalies among the adjacent artificial satellites may be separated from each other by a difference of 120 degrees.
In a range difference measuring system of the above-described conventional positioning system utilizing the artificial satellites, three hyperboloids are drawn as the differences between the four artificial satellites and the observation point, and are determined to be constant, and an intersecting point of the three hyperboloids is obtained. Signals of the range difference measuring system are generated through using an intersatellite communication. In FIG. 2, at the observation point P, the positioning signal output from the artificial satellite A is directly received and the same positioning signal transmitted from the artificial satellite A through the artificial satellite B also is received. At the observation point P, the two positioning signals received from the artificial satellite A through the two routes are compared with each other so measure a time difference AP-ABP and therefore its range difference AP-ABP.
Next, while the positioning signal is transmitted from the artificial satellite A to the GCS, simultaneously, the positioning signal is also transmitted from the artificial satellite A to the artificial satellite B, is turned back from the artificial satellite B to the artificial satellite A and is then transmitted to the GCS through the artificial satellite A. In the GCS, the two positioning signals transmitted from the artificial satellite A via the two routes are compared with each other to measure the range between the artificial satellites A and B, and this is transmitted to the artificial satellite B as telemetry data via the telemetry command channel TC.sub.BG (TT&C channel between the GCS and the artificial satellite B).
As described above, at the observation point P, AP-BP and AP can be known, and thus a range difference (AP.about.BP) can be calculated as follows: EQU (AP.about.BP)=AP-(ASP-AB)
In the same manner as described above, range differences (BP .about.CP) and (CP.about.DP) can be calculated.
At the observation point P, by using these three range differences, the intersecting point of the three hyperboloids having the same range difference can be calculated and thus the position of the observation point P can be obtained.
In this range difference measuring system of the conventional positioning system utilizing the artificial satellites, though frequency (time) accuracy of an original oscillator is required, long term drift and the like of a crystal oscillator can be monitored in the GCS and can be included as the telemetry data into the positioning signals of the artificial satellites to inform to the observer and the observer then corrects the drift.
Further, still another conventional positioning system utilizing the artificial satellites has been proposed. In this case, so as to make an effective positioning at any place on the ground, the number of artificial satellites is increased from 18 to 21 for improving the fix accuracy of the positioning in the GPS (Global Positioning System). This is a countermeasure for the deterioration of the positional fix accuracy by a particular geometrical arrangement of a plurality of artificial satellites in the space varying with the elapse of time, seen from the observer, or for an occurrence of the impossible positional fix. The degree of degradation of the positional fix accuracy determined by the geometrical arrangement of the plurality of artificial satellites is called "GDOP (geometric dilution of precision)". The definition and introduction of the GDOP is disclosed in "GPS--Precise Positioning System by Artificial Satellites", edited by The Institute of Japan Geodesy, p. 131, pp. 140-145, November 1986, and "A Satellite Selection Method and Accuracy for the Global Positioning System" by M. Kihara and T. Okuda, Navigation. Journal of the Institute of Navigation, Vol. 31, No. 1, pp. 8-15, Spring 1984.
The background of the increased of number artificial satellites from 18 to 21 for the improvement of the GDOP is disclosed in "Achieving GPS Integrity and Eliminating Areas of Degraded Performance", by Paul A. Jorgensen, Navigation, Journal of the Institute of Navigation, Vol. 34, No. 4, pp. 297-306, Winter 1987-88, and "Combined Satellite Navigation Systems Could Lead to More Reliable and More Precise Air Navigation", by Randolph Hartman, ICAO Journal, pp. 9-12, March 1991.
In turn, the positioning system utilizing the artificial satellites has a role in navigation of a mobile body on the ground surface, and same time a mobile satellite communication for communicating the mobile body whose position is to be fixed and the GCS (or another communication party connected with-the GCS) in both directions via the artificial satellites also plays an important role. Now, paying attention to an aircraft as a mobile body in particular, a communication system between the aircraft and a ground station plays an important part in the same manner as the navigation system for reaching the desired destination on the basis of the aforementioned positioning.
For a line of sight communication between the aircraft and the ground station, a VHF band is most preferably used. For an over-the-horizen communication, even though existing weak points in propagation characteristics, an HI: band is used as a reliable communication, but this HF band communication has been replaced by an L band satellite communication. Satellite communication in the polar regions is a theme to be solved in Future.
The above-description is disclosed, for example, "Special Comittee on Future Air Navigation Systems", ICAO, Doc. 9524, FANS/4, MAY 1988, and "Report on the Tenth Air Navigation Conference ", ICAO, Doc. 9583, AN-CONF/10, September 1991.
In the case of the aircraft as the mobile body, the use of the satellites of the INMARSAT for mobile satellite communication is known. As regards the communication between the aircraft and the ground, air traffic control communication between a pilot and a controller on the ground, flight control communication between a cockpit and an airline, air service communication between aircraft passenger cabin crew and the airline, and public communication for passengers and the like are given. The mobile satellite communication channel is constructed between an aircraft GCS provided with a communication terminal device installed on the aircraft and the GCS as a base station via the INMARSAT satellites.
The above-described mobile satellite communication of the aircraft as the mobile body is disclosed in "Present Situation of Aeronautical Satellite Communication", by Saitoh, Kawai and Hatayama, KDD Technical Journal, No. 6, pp. 5-9, July 1991.
The conventional positioning systems utilizing the artificial satellites are constructed as described above and suffer from the following problems.
That is, in the conventional positioning system (GPS) utilizing the artificial satellites, the artificial satellites are placed in the orbits with an orbital period of 12 hours and the visible time from the GCS for controlling the artificial satellites is limited. Hence, for keeping the correct time of the positioning signals of the artificial satellites, a standard time generator having a small variation and less frequent correction times is required, and it is necessary to use the complicated and heavy atomic frequency standard (atomic clock) for installing on the artificial satellites. Also, when the artificial satellites are out of sight of the GCS, the condition of the positioning signals of the artificial satellites cannot be monitored in the GCS, and thus the monitoring result of the signal condition cannot be supplied to the observer within the positioning available area in real time.
In the conventional positioning system utilizing the artificial satellites, the artificial satellites are placed in the quasi-geosynchronous altitude orbits having the large orbital inclination angle and the stationary altitude and it is necessary to determine the placing condition for forming the positioning available area of the positioning system in a predetermined region.
Also, in the conventional positioning system utilizing the artificial satellites, the degradation of the positional fix accuracy is sometimes caused by some specific geometrical arrangement of the plurality of artificial satellites, varying with the passage of time, as seen from the observer.
Further, the conventional positioning system utilizing the artificial satellites plays a role in the navigation of the mobile body on the ground surface and same time the mobile body satellite communication for communicating the mobile body whose position is to be fixed and the GCS (or another communication party connected with the GCS) in both the directions via the artificial satellites also plays an important role. However, the conventional mobile satellite communication system is a different system from the positioning system, and when two roles are needed, it is required to use the combination of mobile satellite communication system and positioning system. Further, this is a problem in effective use of the common parts of the systems.